1. Field of the Invention
The present invention relates to an improved and novel lighting circuit of a square-wave lighting type for a discharge lamp, which generates a resonance voltage having a high peak value to compensate for the re-ignition voltage of the discharge lamp that is produced at the time of inverting the polarity of a square wave, thereby preventing frequent lighting failure of the discharge lamp immediately after the activation of the discharge lamp.
2. Description of the Related Art
Recently, downsized metal halide lamps are receiving greater attention as a light source to take the place of an incandescent lamp. A known lighting circuit for a vehicular metal halide lamp is, for example, designed to use a DC power supply in such a way that a DC input voltage after being boosted is convened to an AC voltage of a square waveform-which is in turn applied to the metal halide lamp.
When the polarity of a square wave to be supplied to a lamp is inverted, the re-ignition voltage of the lamp is produced. A circuit to compensate for the re-ignition voltage has been proposed which generates a resonance voltage with a high peak value, thereby preventing the lighting failure, flickering or the like of the lamp.
FIG. 8 shows the essential portions of such a circuit a. A DC power supply section b is provided to acquire the boosted output and/or stepped down output of the supply voltage from a battery (not shown).
A DC-AC converter c, provided at the subsequent stage of the DC power supply section b, converts the output of the DC power supply section b to an AC voltage of a square wave. The DC-AC converter c has a bridge structure comprising semiconductor switch elements.
An inductor d is provided on one (e) of connection lines e and e' which connect the DC power supply section b to the DC-AC converter c.
A capacitor f has one end connected to one end of the inductor d on the DC-AC converter side, and the other end connected to the connection line e'.
An inductor i is provided on one (h) of power supply lines h and h' which connect a metal halide lamp g to the DC-AC converter c.
In this circuit a, the output of the DC power supply section b is convened by the DC-AC converter c to a square-wave voltage which in turn is supplied to the metal halide lamp g via the inductor i. It is however possible to compensate for the re-ignition voltage of the lamp at the time of normal lighting as well as the initial stage of the lighting by using the peak voltage which is produced by the resonance of the inductor d and capacitor f.
When a voltage drop occurs after the LC resonance due to the reaction of the resonance in the above circuit structure, the lamp current temporarily decreases so that the lighting failure of the lamp is likely to occur.
FIG. 9 schematically shows the waveforms at the essential portions at the initial lighting stage of the lamp, and shows the relation between the potential (Va) between the inductor d and capacitor f and the current (IL) flowing across the inductor i. In the diagram, "t1" is the rise point of the square-wave voltage Va, "t2" is the point at which the polarity of IL is inverted, "t3" is the point at which Va rapidly falls close to zero, and "t4" is the point at which IL temporarily falls after t3.
As illustrated, Va temporarily shows the peak by the LC resonance but rapidly falls near zero by the reaction of the resonance, so that a sufficient voltage is not supplied to the lamp. As a result, the current IL which has risen to the peak at t3 after t2 temporarily drops at t4, at which the lighting failure of the lamp is likely to occur.